Recently, increasing demand for high density and high performance electronic devices has spawned the development of complex, stacked, three dimensional (3D) package geometries. These emerging 3D packaging technologies require development of z-axis electrical interconnects for chip-to-chip, chip-to-substrate, and substrate vias. Because the electrical resistance of such interconnects (e.g., formed of conductive organic materials) is critical to the device operation, it is imperative to measure effective resistivity or conductivity of the electrical interconnects accurately. Typically, the effective resistivity is difficult to measure due to its strong dependency on z-thickness of the z-axis electrical interconnects, where interface resistivity plays significant role on the effective resistivity.
Various conventional methods have been used to measure the effective resistivity of z-axis electrical interconnects. One conventional method includes two copper wires placed on a substrate and separated by a small distance. Electrical interconnect, sample material or device under test (DUT) is placed in between the two copper wires and then measured. This method thus measures resistivity or resistance of the DUT across the two wires.
Another conventional method includes a four-point resistivity measurement by spreading DUT materials as a thin film on a glass substrate, and then using a Kelvin-probe setup with four contacts placed side-by-side on the cured thin film so as to measure its electrical resistivity. The measured resistivity generally refers to a bulk volume resistivity along an in-plane (x/y) direction of the DUT. Specifically, with this method, two source probes supply a constant current through the DUT and two high impedance sense probes measure the voltage drop across the DUT. The common current path between the source and sense probes is through the DUT, where current supplied by the source probes generates a relatively high voltage drop, which is seen by the high impedance sense probes.
Conventional methods further include a more advanced four-point resistivity measurement that uses lines patterned on two substrates. The individual substrates are bonded to form a sandwich structure with the DUT located in where the lines overlap or intersect each other. This method is able to measure through-plane electrical resistivity, i.e., along a z-axis direction of the DUT.